Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds such as, for example, the oxides of nitrogen (NOx), unburned hydrocarbons, and particulate matter. Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx and particulate matter emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. Two strategies that may be used to ensure compliance with the regulations are exhaust gas recirculation (EGR) systems and selective catalytic reduction (SCR) systems.
EGR systems are used for controlling emissions of undesirable pollutant gases and particulates during operation of an internal combustion engine. Such systems have proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, trucks, and other on-road machines. EGR systems generally recirculate exhaust gas into an intake air supply of the internal combustion engine. The exhaust gas reintroduced to the engine cylinder reduces the concentration of oxygen in the cylinder, which lowers the maximum combustion temperature, slows the chemical reaction of the combustion process, and decreases the formation of nitrous oxides (NOx). Furthermore, the exhaust gas typically contains unburned hydrocarbons which are burned after reintroduction into the engine cylinder further reducing the emission of undesirable pollutants from the internal combustion engine.
SCR is a process where gaseous or liquid reductant (most commonly urea) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2. One system for selective catalytic reduction is described in U.S. Pat. No. 6,470,676 (the '676 patent), issued to Dölling et al. Specifically, the '676 patent describes a method for catalytic conversion of NOx. A reducing agent is added to the exhaust gas from an engine as a function of the NOx concentration and the operating condition of the engine. If the operating conditions of the engine indicate that less reducing agent is needed, an increased amount of reducing agent is temporarily added to the exhaust which is then stored on the catalyst. When the operating conditions of the engine indicate that more reducing agent is needed, the excess reducing agent that has been stored on the catalyst may be utilized to convert the added NOx until more reducing agent can be added to the system.
While EGR systems may be effective at reducing undesirable pollutants and particulates, they may cause increased cylinder pressure and fuel consumption at high loads, and may increase exhaust temperature requiring relatively large heat exchangers to cool the exhaust before reintroduction into the engine. Furthermore, while prior art systems may be effective at reducing NOx emissions, they may not be the most effective choice across a wide range of engine loads. At low loads, the temperature of the exhaust gas is low and the efficiency of the SCR catalyst is reduced. Furthermore the extensive use of a reducing agent would require a large amounts of storage or alternatively a reduced operating time.
The disclosed exhaust aftertreatment system is directed to improving prior art systems.